Column-stabilized offshore vessel

ABSTRACT

The invention relates to a semi-submersible vessel designed to operate in water depths of 200 to 10,000 feet. The vessel has a ring pontoon with an outer and inner configuration of the pontoon forming a triangle. Three corner columns are fixedly attached at their lower ends to the ring pontoon. The upper ends of the stabilizing columns carry an upper deck designed to support drilling and production operations. The vessel uses no diagonal or horizontal braces, nor auxiliary supporting columns. The triangular shape of the vessel minimizes environmental forces acting on the vessel, thus improving its stability at any operational draft. The vessel can be fully constructed and outfitted quay (dock) side.

BACKGROUND OF THE INVENTION

[0001] The invention relates to an offshore drilling and production vessels, and more particularly to a semi-submersible vessel for conducting offshore operations in water depths from 60 feet to 10,000 feet, such as the Gulf of Mexico, the North Sea and the like.

[0002] In recent years, oil and gas drilling and production operations have been conducted at increasingly greater distance from the shoreline, making oil and gas prospecting more expensive. Small companies or individual operators find it difficult to compete with major oil companies partly due to increased fabrication and deployment costs associated with large semi-submersible units or jack-up rigs. Usually, such units are deployed at rich reserve sites, where the prospect of substantial production amount is anticipated.

[0003] However, there exists a substantial number of marginal field production in deep water, for instance in depths from 3,500 to 7,500 feet. It is not very cost effective to deploy a conventional permanently positioned unit in a marginal field. As a result, the operators use mobile offshore production units (MOPUs) to operate in such environment. At present, the total worldwide fleet of MOPUs is comprised of approximately 135 floating units. The present invention is directed to providing a more cost-effective solution to marginal field production in deep water.

[0004] Various designs of floating structures are known in the art for supporting the mineral exploration and production operations, as well as providing living accommodations to the crew and storage for the necessary equipment. For instance, in deep waters, over 7500 feet, it is conventional to deploy floating semi-submersible vessels, as opposed to fixed bottom anchored structures.

[0005] The majority of semi-submersible vessels utilize buoyant pontoons, or lower hulls that support a plurality of vertically extending columns, the upper portions of which carry a working platform with one or more decks. Some semi-submersible vessels have a single caisson, or column, usually denoted as a buoy, while others use three or more columns extended upwardly from buoyant pontoons.

[0006] In many such structures, vertical and/or diagonal braces are used between the columns, the braces contributing to the water plane area of the vessel. The braces usually have much smaller diameters than those of the supporting columns and are therefore more vulnerable to the environmental and mechanical damage. If the connecting braces are damaged, the entire structure becomes jeopardized.

[0007] One example of a three-column structure is shown in U.S. Pat. No. 3,246,476 issued on Apr. 19, 1966 to Wolff for “Submersible Vessel for Submarine Operations.” The semi-submersible vessel of the '476 patent, in some of its embodiments, has three stabilizing columns connected at the bottom to a triangular base. The base has connecting braces joining the main base members at midsections and forming a triangular inner base. Auxiliary vertical columns are provided for supporting the upper deck.

[0008] There also exist numerous designs of semi-submersible vessels using diagonal braces in addition to horizontal stays. These tend to reinforce the support structure of the platforms and resist destructive forces of the ocean waves. One of the designs using diagonal braces in a ring pontoon is shown in U.S. Pat. No. 6,015,245 to Frimm et al. The '245 patent has six columns interconnected by a number of pontoons forming a generally rectangular base. One of the disadvantages of the additional columns and braces is increase in the water plane area of the vessel, which adversely affects the weight, wave resistance and overall cost of the vessel.

[0009] Another consideration that is taken into account when designing semi-submersible vessels is resistance of the vessels to motions induced by waves. The vessels must have sufficient capability to withstand wave motions to allow the mineral exploration and production operations to be carried out in safety with minimal downtime to maximize efficiency. The present invention contemplates provision of a column-stabilized semi-submersible vessel with improved safety features, cost efficiency and resistance to wave induced motions.

SUMMARY OF THE INVENTION

[0010] It is, therefore, an object of the present invention to provide a semi-submersible vessel with improved safety features.

[0011] It is another object of the present invention to provide a column-stabilized semi-submersible vessel with a ring pontoon.

[0012] It is a further object of the present invention to provide a semi-submersible vessel that uses no additional braces assuring minimal increase in water plane area, while resisting spreading and twisting forces acting on the global structure.

[0013] These and other objects of the invention are achieved through a provision of a semi-submersible vessel for use in water depths from 200 to 10,000 feet and particularly advantageous for use in depths of 3,500 to 7,500 feet. The vessel comprises three vertical stabilizing columns connected at their lower ends to a ring pontoon. The pontoon has three sections that form a triangle. The corners of the triangle are attached to the vertical columns. The upper ends of the columns carry a working platform or deck, from which mineral exploration and production operations are conducted.

[0014] The vessel has a relatively small water plane area since it does not require any diagonal braces to reinforce the support column structure. The ring pontoon resists column twisting that may be caused by wave and current forces. In addition to added resistance to spreading and twisting forces acting on the overall structure the ring pontoon offers cost efficiency in fabrication and deployment. Where most floaters need heavy lift vessels in order to integrate the topside platform with the hull, the semi-submersible vessel of the present invention it does not require offshore integration in its capital cost range.

[0015] To minimize drag forces induced by wave motions, the columns are provided with outwardly convex, rounded corners. This design reduces drag force transmitted when mooring or moving the vessel while maximizing the storage for an optimum design. The pontoon hulls may have compartments for storing ballast, drill water and other necessary supplies, while the columns may house reserve mud tanks, ballast tanks, force air ventilation supply and other necessary machinery and equipment. Living quarters and service equipment may be mounted on the platform.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Reference will now be made to the drawings, wherein like parts are designated by like numerals, and wherein FIG. 1 is a perspective view of the semi-submersible vessel in accordance with the present invention.

[0017]FIG. 2 is an outboard profile illustration of the vessel in accordance with the present invention.

[0018]FIG. 3 is an outboard profile of the vessel of the present invention taken at a different angle from the view of FIG. 2.

[0019]FIG. 4 is a schematic view of the ring pontoon of the semi-submersible vessel of the present invention.

[0020]FIG. 5 is a schematic view showing an example of connection of the bottom of the columns to the ring pontoon; and

[0021]FIG. 6 is a cross-sectional view of a column taken along lines 6—6 of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] Turning now to the drawings in more detail, numeral 10 designates the semi-submersible vessel of the present invention. The vessel comprises a buoyant ring pontoon 12 having sections 14, 16, and 18. The pontoon sections are connected together to form a triangle, which may be an equilateral triangle or other geometric shaped triangle. The outer perimeter of the ring pontoon 12 defines a generally triangular shape. A central open space 20 defines an inner perimeter of the pontoon 12 and follows in configuration the outer configuration of the pontoon 12. Both the outer perimeter of the ring pontoon 12 and the inner perimeter of the ring pontoon 12 form an equilateral triangle. Each pontoon section 14, 16 and 18 has a generally rectangular cross section; each section is defined by a top plate 22, 24 and 26, respectively, a pair of vertical side plates 28, 30, 32, 34, 36 and 38, respectively and a bottom plate (not shown).

[0023] The corners of the triangular pontoon have flat vertical plates 40, 42 and 44 for attaching to respective mating surfaces of the columns, as will be described below. The pontoon sections may be divided into a plurality of watertight compartments for accommodating ballast.

[0024] Secured to the corners 40, 42 and 44 of the pontoon sections 14, 16 and 18 are vertical stabilizing columns 50, 52 and 54. The columns 50, 52 and 54 extend at right angles to the horizontal axis of the ring pontoon 12. Each column 50, 52 and 54 has a generally square cross-section (see FIG. 6).

[0025] To improve the drag characteristics of the vessel, that is to minimize drag, the exterior corners of each column is formed with rounded vertical corners 60, and 62. The rounded corners 60 and 62 are attached to sides 65, 66 and 67 of each column by welding and the like. The side 64 of each column 50, 52 and 54 contacts a respective mating side 40, 42, or 44 of the ring pontoon 12.

[0026] The radius of the outward curvature may be ¼ or greater of the width of the generally square column. Such rounded corners minimize drag forces, while still providing the necessary strength and storage to the corner columns. Additionally, the vertical flat panels of the columns are less expensive to fabricate as compared to conventional round columns of the prior designs.

[0027] The side 64 of each of the columns 50, 52 and 54, is made of a flat plate to allow mating attachment of the column to a corner of the ring pontoon 12, as shown in FIG. 5.

[0028] In the preferred embodiment, the buoyant pontoon 12 is intercostal to the columns, i.e. it is made of separate parts, not continues with the columns 50, 52 and 54. As a result, construction of the vessel is simplified and made more cost-effective. The sections 14, 16 and 18 of the pontoon 12 close the area between the three columns 50, 52 and 54 to provide the necessary buoyancy and strength.

[0029] The upper portions of the columns 50, 52 and 54 carry a platform 70 adapted for supporting mineral exploration and production operations. During offshore operations, parts of the columns 50, 52 and 54, as well as the ring pontoon 12 are submerged below the water surface to an operational draft, while the upper portions of the columns 50, 52, 54 and the platform 70 are elevated to a level above maximum expected wave for a particular location. The ballast chambers in the pontoon 12 provide the necessary room for introducing ballast, for example, sea water to partially submerge the structure.

[0030] The platform 70 is of a modular type and is attached to the upper ends of the columns 50, 52 and 54 in such a way that the entire deck could be removed for replacement by a different type of deck, designed for other purposes. For instance, the platform 70 may be equipped for oil recovery, then substituted by a platform suitable for gas production, work over operations or drilling. Generally, the hull is independent from topside to allow easy changing of the deck for different job requirements.

[0031] The main or upper deck platform 70 is a simple grillage beam structure designed to support the processing equipment and facilities. This approach eliminates the need for special equipment support structure and allows individual items of equipment to be simply put in place and connected by piping and electrical line to achieve the process facility. Such design saves both time and cost.

[0032] The operational draft of the vessel 10 is generally higher than a survival draft. When the vessel 10 encounters particularly harsh conditions, wave and hydrostatic forces act on the pontoon and on the columns, causing the vessel 10 to move vertically and angularly, subjecting the vessel to heave and pitch motions of the wave. These forces which cause vertical and angular motions of the vessel result in the vessel shifting in relation to a vertical axis. The columns 50, 52 and 54, being relatively large in cross-section help stabilize the vessel and to reduce wave-induced motions acting on the vessel.

[0033] Mounted on the main deck 72 of the platform 70 are a pair of winches 74, one at each column 50 and 24 (FIG. 1). Since the need for the permanent winches, chain lockers, chain and wire line storage has been eliminated through the use of pre-set mooring technology, the vessel 10 has lower operational costs, which advantageously affects the cost of the minerals recovered with the use of the vessel 10.

[0034] The vessel 10 provides a facility for conducting mineral exploration in relatively deep waters of up to 10,000 feet. The vessel 10 is equipped with at least one crane 76, although more than one crane is usually provided. A heliport 78 is erected in one corner of the triangularly-shaped platform 70. The helicopter landing pad is used for delivering personnel and supplies to the vessel 10 when in operation. Drilling and production operations are conducted through the platform 70 in a manner well known to those skilled in the art.

[0035] Flexible lines (not shown) from sub sea oil and gas wells may be connected from the sub sea wellhead to the pontoon sections, or lower hulls 14, 16 and 18. Connection of the flexible lines at the lower hull allows existing technology to be effectively utilized and by connecting below the total vertical center of gravity, additional stability capability is not required. The vessel 10 must only provide sufficient buoyancy to counteract the platform weight, the weight of the process facility mounted on the main deck 72, the mooring line loads and the flexible line loads from the wells and the export or sales line(s). This design reduces the amount of displacement and the amount of steel required to fabricate the vessel 10.

[0036] The compartments formed in the ring pontoon 12, in addition to holding salt water ballast, may be equipped for housing pumps, storing drill water, diesel oil and other necessary materials. The columns 50, 52 and 54 may be also compartmentalized to hold reserve mud tanks, brine tanks, base oil tanks, ballast tanks, vents, ventilation supplies and return pipes for lower hull and column machinery compartments. Water is allowed to enter the ballast chambers by gravity and may be expelled from the chambers by compressed gas or air.

[0037] Other equipment necessary for offshore operations, such as, sewerage treatment unit, portable water tanks, storage tanks can be positioned in other compartments within the columns 50, 52 and 54. The upper hull, or platform 70 structure may be designed to house drilling and ship service equipment, power generation and storage for liquid mud, sacks, and other variable materials and equipment.

[0038] The vessel 10 uses no diagonal or horizontal braces, or intermediate columns, instead relying on the ring pontoon and the three large columns to provide the displacement, stability and strength to the vessel having improved motion characteristics. There are no on-deck mooring winches; mooring is accomplished with suction piles that are pre-set at location. As a result, the vessel 10 is very mobile and may be leased/rented by an operator and brought to a location with minimum expenditure for deployment. Once the site is depleted, the vessel 10 may be easily moved to another desired location.

[0039] The vessel 10 may be deployed at varying depths, from 200 to 10,000 feet of water. It is envisioned, however, that the vessel 10 will find its greater application for use in 3,500 to 7,500 feet water depth. The column height may be up to 110 feet, with waterline passing at about 60 to 80 feet.

[0040] The triangular shape of the vessel 10 minimizes environmental forces acting on the vessel, thus improving its stability at any operational draft. The vessel can be fully constructed and outfitted quay (dock) side. The design lends itself to easy size modification; it may be scaled to accommodate the more demanding processing requirements.

[0041] The vessel 10 has adequate stability to be floated to the desired location, where it is partially submerged moored and left for both normal operations and for extreme environmental conditions. The vessel may be towed to another location at a deep or shallow water draft since it is designed to be transported like a ship. It is also capable of being dry towed to other locations throughout the world if more cost efficient to the owner/operator. In comparison to other floating structures, for instance Spar-type, it has better design characteristics for relocation and re-use at a new site.

[0042] Many changes and modifications can be made in the design of the present invention without departing from the spirit thereof. We, therefore, pray that our rights to the present invention be limited only by the scope of the appended claims. 

We claim:
 1. A semi-submersible vessel, comprising: a buoyant generally triangularly-shaped pontoon; a plurality of vertical stabilizing columns fixedly attached to said pontoon, each column having an intercostal connection with a respective corner of said pontoon; and a platform supported by upper ends of said columns for conducting offshore operations therefrom.
 2. The vessel of claim 1, wherein said pontoon is a ring pontoon.
 3. The vessel of claim 2, wherein an outer perimeter of said ring pontoon forms a triangle.
 4. The vessel of claim 2, wherein an inner perimeter of said ring pontoon forms a triangle.
 5. The vessel of claim 1, where said pontoon comprises a plurality of pontoon sections, each end of each pontoon section being fixedly attached to an adjoining pontoon section and to at least one of said stabilizing columns.
 6. The vessel of claim 5, wherein each of said columns is attached to said pontoon along at least one side.
 7. The vessel of claim 5, wherein each of said pontoon section has a generally rectangular cross section.
 8. The vessel of claim 1, wherein each of said stabilizing columns has a generally square cross-section, and wherein outer corners of each of said stabilizing columns is outwardly convex to reduce drag forces acting on the vessel.
 9. A semi-submersible vessel, comprising: a buoyant ring pontoon having an outer perimeter defining a triangle; a plurality of vertical stabilizing columns fixedly attached to said pontoon, each column being connected along at least one side to an intersecting corner of the ring pontoon sides; and a platform supported by upper ends of said columns for conducting offshore operations therefrom.
 10. The vessel of claim 9, wherein the ring pontoon has a non-continuous connection to each of said stabilizing columns.
 11. The vessel of claim 9, where said pontoon comprises a plurality of pontoon sections, each end of each pontoon section being fixedly attached to an adjoining pontoon section and to at least one of said stabilizing columns.
 12. The vessel of claim 9, wherein an inner perimeter of said ring pontoon forms a triangle.
 13. The vessel of claim 9, wherein each of said stabilizing columns has a generally square cross-section, and wherein outer corners of each of said stabilizing columns are outwardly convex to reduce drag forces acting on the vessel.
 14. The vessel of claim 11, wherein each of said pontoon sections has a generally rectangular cross-section.
 15. The vessel of claim 9, wherein each of said columns is attached to said pontoon along at least one side.
 16. The vessel of claim 9, wherein each of said columns is attached to said pontoon along more than one side. 